1. Field of the Invention
The present invention is directed to a system and method for detecting or monitoring of action potential firing in a target nerve and based on such feedback information optimizing stimulating therapy for an external nerve stimulating device.
2. Description of Related Art
External electrical stimulating devices are well known and used for stimulating nerves such as the pudendal nerve, sacral nerve, median nerve, ulnar nerve, transpalmar median nerve, or any other nerve. Nerve stimulation is a process by which a specific target nerve is stimulated using external electrodes that generate electrical pulses having a particular frequency, amplitude and waveform. A nerve cell can be excited by increasing the electrical charge within the nerve, thus increasing the membrane potential inside the nerve with respect to the surrounding extracellular fluid. An action potential (AP) occurs when a neuron sends information down an axon or fiber. The action potential is an electrical impulse or spike that is created by a depolarizing current and propagates down the length of the axon. Electrical stimulation from an external stimulation device can trigger the firing of an action potential.
A threshold stimulus intensity is defined as that required to trigger an action potential. The action potential is an all or nothing principle. That is, the action potential will only be fired by the neuron and the signal propagated along the axon if the neuron reaches this threshold stimulus intensity. The amplitude of an action potential for a particular nerve is substantially constant and independent of the amount of stimulus current. So long as the threshold stimulus intensity is reached, the neuron will fire an action potential having a substantially constant or fixed amplitude. Instead, the frequency of the action potential is what determines the intensity of the stimulus. The information in the nervous system is therefore coded by frequency of firing rather than the amplitude of the action potential.
External nerve stimulating devices or stimulators are advantageous due to their relatively inexpensive cost of manufacture in comparison to implantable stimulating devices and noninvasive ability to stimulate a nerve without requiring surgery or implantation. However, because the stimulating device is external to the body, there exists the possibility of improper alignment or positioning of the stimulating device thereby undesirably stimulating a non-target nerve without stimulation of the target nerve. If the electrical stimulating current is too large, it is possible to undesirably stimulate non-target nerves simultaneously with the target nerve. In addition, the amplitude of the stimulation current if too large may cause undesirable physical discomfort, temporary or even permanent tissue damage to surrounding tissue, or expend too much energy from a power source limited due to its relatively small size. On the other hand, if the amplitude of the stimulation current is too small the stimulator may not be effective in stimulating the target nerve.
It is therefore desirable to develop an improved method and system for optimizing stimulation therapy by monitoring firing of an action potential and providing a feedback signal for adjusting of stimulating signal parameters.